Reductions of 1,3,5-Triboracyclohexanes and Related 1,3,5-Triboraalkanes with Alkali Metals to Trishomoaromatic Aggregates - Tetrameric Lithium and Sodium, and Polymeric Sodium and Potassium Trishomoaromatic Cyclotriborate Compounds

• Published in: European journal of inorganic chemistry Vol. 2001; no. 8; pp. 1949 - 1956
• Main Authors: Lößlein, Wolfgang, Pritzkow, Hans, von Ragué Schleyer, Paul, Schmitz, Lawrence R., Siebert, Walter
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag GmbH 01.08.2001; WILEY‐VCH Verlag GmbH
• Subjects: Alkali metals; Aromaticity; Boron; NMR spectroscopy
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/1099-0682(200108)2001:8<1949::AID-EJIC1949>3.0.CO;2-4

Hexamethyl‐1,3,5‐triboracyclohexane can be reduced by alkali metals in donor solvents. Two electrons are added to the combination of p‐orbitals at the boron centers forming trishomoaromatic dianions. X‐ray structure analyses show a variety of arrangements, depending on the alkali metal: Tetrameric aggregates with lithium, a polymeric, meander‐like chain or a complex tetramer with sodium, and a zigzag‐chain polymer with potassium. The set of dianions show characteristic NMR chemical shifts in the high‐field region, but do not obey a monotonous order with the counterions Li+ to Cs+. Lithium is out of line because of its exceptionally low electronegativity. IGLO chemical shift calculations agree very well with the experimentally observed data. The computed Nucleus‐Independent Chemical Shift (NICS) values prove the strong homoaromatic character of the reduced triboraheterocycles. Compared with isoelectronic analogues the reduced triboraheterocycle shows a remarkable stabilization energy towards the corresponding, non‐aromatic species.